As a process of injection molding foamable plastics, Japanese Patent Kokai Sho 62-246710 discloses an injection molding process wherein a mold to be connected to the injection port of an injection molding machine consists of a stationary mold part and a movable mold part, a cavity formed therebetween can be enlarged and reduced by moving the movable mold part, and before a foaming agent is injected into the mold from the injection molding machine, the cavity is enlarged to a predetermined size by making the movable mold part go backward.
Further, Japanese Patent Kokai Hei 4-214311 discloses an injection compression molding process wherein a mold relatively movable to the direction of enlarging or reducing the cavity volume is placed in the position of reducing the predetermined cavity volume, the mold is moved to the direction of enlarging the cavity volume while injecting a foamable molten resin into the cavity by maintaining the pressure of preventing the resin from foaming, the resin is compressed by moving the mold to the direction of reducing the cavity volume, the surface of the resin is cooled to solidify, the resin pressure is lowered to that of initiating the foaming by moving again the mold to the direction of enlarging the cavity volume, thereby foaming the resin within the cavity, and after cooling, a molded article is removed.
In such prior processes, however, the problem remains that the molded article of good appearance is not produced. The cause in the former process will be considered as mentioned below. Since the cavity volume is enlarged at the same time as the injection, a cell collapse occurs at the surface portion of the molded article and the surface having a non- or low-foamed, dense cell structure is not obtained. In case of using a mold having the cavity structure with a thin-wall at the position away from the gate, it is difficult to completely fill the mold. The cause in the latter process will be considered as mentioned below. In case of using the mold having the cavity structure with a thin-wall at the position away from the gate (e.g. the mold for the molded article having a large flat area, ribs and bosses) and a conventional molding machine, it is difficult to maintain the pressure of the foamable plastic so as to prevent it from foaming. If the injection is performed at a high-speed and a high-pressure using a large molding machine or a counter-pressure process is used in combination therewith, the foamable plastic can be, of course, maintained at the pressure of preventing it from foaming. However, the problem remains on economy.
Further, the prior processes have the problems that in the production of the molded article having a large flat area, a thin-walled rib and wall surface or boss at the end portion, the resin cannot be completely filled to the end portion of the molded article (what is called "short shot"), which results in the occurrence of defective and no production of the molded article having a dense surface and good appearance.
The cause of the above problem will be considered as mentioned below. In either of the above prior processes, the cavity is enlarged at the same time as injection or in the middle of injection, that is, the volume of the cavity to be filled is increased in the middle of resin feeding. If said operation is conducted with the cavity having a large flat area, the case may occur where the feed rate of resin cannot follow the rate of increasing the cavity volume. In such situation, it is very difficult that the pressure of the resin to be fed within the cavity is kept constant and the resin cannot be prevented from the initiation of foaming. With the cavity having the shape as above in which the resin feed pressure more than a predetermined value is required for the filling to the end portion, short shot is inevitable. Even if the cavity is compressed again after it was enlarged, most of the resin foaming is already completed at that time. In the step of compressing the cavity, therefore, the cell produced by foaming is only compressed and the effect of filling a fluid resin to the end portion cannot be expected.
For the same reasons as mentioned above, a reduction of resin pressure within the cavity will promote foaming in the middle of feeding the resin, so that the cell produced by foaming will rupture successively at the front portion of the feeding resin, leaving traces on the surface of the molded article, with no production of the molded article of good appearance.
To prevent the reduction of pressure as mentioned above, a counter-pressure process has been employed together wherein a compressed gas is previously sealed into a mold to keep constant the pressure at the front portion of the fluid resin. Even in such a case, there is a phenomenon that a resin flow apparently stops or goes backward at the flat area of the cavity in connection with the feed rate of the resin and the speed of increasing the cavity volume, so that the trace or mark appears on the surface of the molded article, leading to poor appearance of said article.
To remove such drawbacks of prior art processes, there is no other way but to limit the shape of the molded article to bar, column or the form having sufficient thickness to the thickness direction relative to the projection area of the flat portion. That is, the production of a good molded article by the processes comprising the step of enlarging the cavity at the same time as injection or in the middle of injection is under the restraint that the shape of the molded article is restricted to the narrow range.
Thus, the present invention is devised in view of such drawbacks of the prior art and the object of the invention is to provide an injection molding process of producing a molded plastic foam article having a light weight, good appearance and good stiffness, the surface of the molded article comprising a layer having a non- or low-foamed, dense cellular structure and the inside of the molded article comprising a high-foamed layer, by means of conventional injection molding machine using a mold having the cavity structure with a thin-wall at the position away from the gate.